The strategic value of RLEP lunar polar
exploration

White paper DRAFT v. 2,April 19,
2006

Ben Bussey, Jeff Plescia, and Paul
Spudis, APL

Strategy

The Vision for Space Exploration
calls for a return to the Moon with robots and humans.A key objective is to learn how to use lunar
resources to support the long-term presence of humans on the Moon and to enable
further exploration.This objective
implies extended presence on the surface of the Moon, the development of a
significant infrastructure, and the exploitation of local resources to the
greatest extent possible.A constraint
on the Vision is the philosophy of “pay-as-you-go.”To facilitate pay-as-you-go and reduce risk, you need to “know
before you go.”

There are a number of decisions
that need to be made regarding the lunar return architecture.One key decision is what resources are most
important and how they will be used.For example, oxygen and water may only be required to make up for losses
to a close life support system.Alternatively, these resources may be harvested for the production of
fuel for cislunar operations as well for journeys farther afield.The latter option is a necessity to enable
pay-as-you-go, permanent presence on the Moon and further Solar System
exploration.

ISRU

Among the first questions to be
answered are the extent to which ISRU is viable, which resources will be used,
and how they will be extracted.Hydrogen and oxygen, two important resources, can be found virtually anywhere
on the Moon.Over most of the Moon,
hydrogen is found in very low abundance (less than 100 ppm) from solar wind
implantation and oxygen is about 45% of the lunar soil by weight.

There are three issues with
respect to exploitation of lunar resources: 1) the energy required to extract
the substance of interest; 2) the efficiency and complexity of the process of
extraction (e.g., batch vs. continuous processing); and 3) the infrastructure
needed on the Moon to establish resource production (e.g., mass needed on the
Moon).A number of processes have been
identified to extract O2 from the both the mare and highlands
regolith; they have varying energy requirements, production efficiencies, and
infrastructure.Most processes are
inefficient (less than a few percent yield) and require significant energy
(tens of kWh/kg).Some are
feedstock-sensitive, e.g., ilmenite reduction requires high-Ti mare
regolith.Demonstrations of these
various techniques can be done within the constraints of a robotic mission, but
most would need significant infrastructure for industrial-scale production
(e.g., tens of metric tons/year, as in the production of propellant.)

The Known Moon

The
Apollo and Luna missions provided an excellent database for understanding the
chemical and physical properties of the equatorial lunar regolith. Thus, our
understanding of the requirements, feedstock, and extraction methodologies for
a variety of resource processing schemes is reasonably complete for equatorial
and mid-latitude regions of the Moon.Some issues remain with respect to handling significant volumes of
granular material; this information could be addressed by RLEP or early in the
series of human lunar return missions.

Energy costs for extracting both O2
and H2 from equatorial soils is given in Table 1.Although taking advantage of abundant “free”
solar thermal energy (available half the time at the lunar equator), extracting
H2 from typical lunar soils is a very energy intensive
exercise.Because no process is totally
efficient, the higher the energy input, the more waste heat that must be
rejected or transferred.So while solar
thermal energy is “free,”the mass of
radiators and other supporting equipment grows exponentially as energy and
power use expands.For this reason, all
previous studies of ISRU have identified the availability of water ice and
other volatiles to enable low energy resource extraction as a key question that
must be answered before a cost/benefit analysis of ISRU can be completed.Once an ISRU industry is started on the
Moon, higher energy processes may be developed and gradually introduced using
lunar materials as the source of its high mass components (retorts, pipes,
etc.)

The Unknown Moon

The poles of the Moon are both
different and largely unknown.Permanently shadowed regions may hold significant quantities of water
ice (and other volatiles) mixed with the regolith.Water ice present in significant amounts (greater than 1-2 wt. %)
would be a valuable resource.Water
derived from ice can be electrolyzed to produce both H2 and O2
in a relatively low energy process (Table 1).The energy savings, in terms of ore reduction, would be significant over
the production of both these substances at the equator.

We do not now understand whether
water ice is actually present at the poles and if so, in what quantities and
manner it is distributed.If present,
ice would provide an important resource that might be extracted with relatively
low amounts of energy.Water
electrolysis is a mature terrestrial technology used extensively at industrial
scales (e.g., submarines.)Extracting
oxygen from silicates has rarely been done at large scales, so an entirely new
industrial base would have to be created to do that on the Moon.(It is done in the aluminum-smelting
industry, which is one of the most energy intensive terrestrial mining
processes.)So while theoretically
possible, oxygen production by breaking silicate and oxide bonds is much more
difficult.

While
hydrogen could be extracted from the solar wind in the regolith, such
processing requires a large investment in initial surface infrastructure.Previous studies of the economics of lunar
ISRU suggest that finding the highest concentration of volatiles closest in
distance to permanently lit areas is a high priority for robotic precursor
missions.Finding such deposits allow
“bootstrapping” of capability from RLEP-scale infrastructure, giving us
leveraging capability early in a program of lunar return.In terrestrial mining, the easiest
accessible ore allows the industry to get started, otherwise costs would be
prohibitive.Once started, it permits
lower grade and less accessible ores to be extracted because the initial production
produced enough wealth to finance the later, more difficult steps.The industrial revolution in Britain started
using wood for charcoal.Once the
easily harvested wood was gone, coal mining started, necessitating the
development of the Watt steam engine to pump water out of the mines, which
required more use of coal for more steam engines.The same thing happened in the development of oil and gas; the
first oil wells (e.g., Drake in PA in 1859) were shallow (depths of 50 feet or
so) but that started an industry to produce kerosene to replace depleted whale
oil, whose growth allowed deeper drilling and more exploration.So while water is desired not to be on the
“critical path” to human lunar return, in fact, it is – because of the
economics of mining and processing.Something too difficult and expensive to do (such as cracking oxygen out
of rock) will not be as enabling.

RLEP

One of the objectives of a robotic
program is to provide data necessary for the definition and decision on
architectures for the human return to the Moon (know before you go).In the case of lunar resources, RLEP should
provide critical knowledge to understand whether water ice is present at the
poles, its form and concentration, and provide a basis for understanding what would
be required to process the regolith to extract it.An RLEP mission to explore a permanently shadowed crater (e.g.,
Shackleton) would provide such data if suitably designed and instrumented.Exploration of a permanently dark, coldcrater is challenging, but represents a step
that must be taken because of the huge potential benefit polar water could
provide.

The proximity of the dark areas of
the poles to areas of permanent sunlight could make mining feasible by
providing a local power source capable of being transmitted by several means
(e.g., cable transmission, beaming, recharging by return to sunlight).Space systems are designed routinely to
operate in eclipse periods provided they have adequate power storage and
thermal design.In space, it’s often
easier and requires less power to stay warm in the cold than to keep cool under
high temperatures, so while challenging, working in the polar darkness is a
problem well within the engineering state-of-the-art.

Once an RLEP mission has
determined the presence, form, distribution and concentration of polar ice, a
decision can be made as which ore (equatorial mare regolith or polar ice) is
the most viable from a programmatic perspective.The problem is more complicated than a simple comparison of the energy
required for break the O-Si or H-O bonds.Equipment and energy will be required to mine the regolith, move the
regolith to a processing location, and store the products.It may be possible to extract polar
volatiles in situ (by mobilizing subsurface volatiles), akin to a gas or molten
sulfur well on Earth.Such a technique
would require no significant soil movement and handling; no other identified
lunar ISRU process allows this.These
factors are dependant on the nature of the deposit and distances involved
(which is a partly function of ore grade).It is the sum total of these considerations that permit a decision to be
made in an informed manner.

After a mission to characterize
the potential of polar resources and a decision regarding which ore to exploit,
subsequent RLEP missions should demonstrate regolith mining (certify
technologies to collect, move and process regolith), conduct extraction
experiments (processes, efficiencies, possible problems), and experiment with
conversion and storage techniques and processes.These demonstrations would allow us to fully understand the
advantages and problems of selected methods of resource extraction and make
informed decisions on the which techniques to pursue.

Poles vs. Low Latitude

In addition to ore grade,
environmental conditions drive surface operations.Operations at the equator can be conducted only in two-week
increments, unless a suitable power source is provided to permit night
operations.(Nuclear) There are also
large temperature extremes between noon and midnight (250° C temperature differential).In the lit areas at the poles, the mean surface temperature is
lower but relatively constant (~ –50°± 10°C).Sunlight is always at a grazing incidence
and there are areas where sunlight is present for most of the time with only
relatively brief periods of eclipse.

We can choose to go to the equator
today; we have significant knowledge of the Moon and its materials in
equatorial regions to design and outfit a human lunar outpost now.We have enough samples and detailed surface
information to design an ISRU operational plan; we know its level of difficulty
and the likely costs of producing a given amount of product per unit time.If such a path is chosen, there is no need
for any RLEP mission whatsoever.But,
such a decision now, while avoiding near-term RLEP costs, would result in
higher risk and possibly much higher overall cost to the implementation of ISRU
in human lunar return over the program lifetime.

Summary and Conclusions

Harvesting polar resources may give huge leverage to
lunar ISRU.Both H2 and
O2 are obtained for comparable amounts of energy input that
yield O2 only at the equator.While H2 is only a small percent of the mass of water,
bringing hydrogen from Earth in liquid form requires much larger volume
with associated costs and mass.Previous studies have shown that this limits the use of ISRU
largely for local life support replenishment.Without H2 from the Moon, lunar resourcescannot be used as a base to expand human
activity in the Solar System, a key goal of the VSE.

Polar
ice may be most suitable for continuous processing; although we don’t know
this, we DO know that equatorial soils are NOT amenable to it.

Early
propellant production can leverage cislunar transport significantly.Potentially could pay for itself after
a few years of operations (a small facility could produce ~ 50 mT/year,
the typical payload mass of a cargo-based LSAM.)Economic studies suggest this is only possible if there is
an accessible source of lunar hydrogen.

Because
both polar environment and deposits are potentially enabling, an effort
should be made to answer the questions about their utility.

This
is a good architectural objective for RLEP.Use early landed missions to address whether such is worth
pursuing.

There
are significant consequences to NOT finding out about polar conditions;
you are driven down a road of known difficulty (and it is considerable.)Likely non-continuous processing,
little or no H2 production, ISRU to remain experimental (rather
than productive) for a much longer time.

Processes
and infrastructure developed for equatorial O2 mining may be
only marginally applicable to polar needs, if the poles are later found to
be desirable.

Over
the next six years (2006-2012), NASA will spend about $100B.An RLEP program of 1-2% of that total
is not unreasonable, considering that a goal of the VSE (returning to the
Moon to stay) is a prime objective.NASA should be able to spend a few percent of its total budget to address
a central issue in its primary mission.An RLEP program that produces a few tens of metric tons of
pre-emplaced water and other products (e.g., radiation shielding) more
than pays for itself if it reduces the total number of required LSAM
missions over the life of the program by one or two.Today the combined space lift
capability of the United States and Russia is used to maintain a
continuous human presence in low Earth orbit and much of the transported mass
is water.Finding accessible water
on the Moon would be the breakthrough that enables permanent self
sufficiency offEarth, a key goal
of the Vision.